Thyroid hormones affect the growth, differentiation and development of tissues. However, the detailed mechanism of their action still remains uncertain. Since thyroid hormones are bound to proteins when transported through the general circulation, when at the cell membrane or in the cell nucleus, identification of the molecular features responsible for binding is of great importance. Many pharmacological agents are found to influence the distribution and action of thyroid hormones by either inhibiting binding to specific plasma proteins or by altering the number or affinity of cellular binding sites. Thus, the observation that thyroid hormones bind to proteins with different affinities suggests that it may be possible to design compounds that can influence receptor binding and hormonal function in specific ways. The primary goal of the parent project is to achieve, by analysis of protein crystal structure data, a molecular level understanding of the biological mechanisms regulating thyroid hormone action, in particular binding to the human serum transport protein transthyretin (TTR). From the crystal structure determinations of human transthyretin complexed with thyroxine (T4) and its metabolites and competitors can be derived the (1) molecular level details of TTR hormone binding interactions, (2) binding site interactions of competitors, (3) design of antagonist analogues which selectively compete for hormone binding sites on TTR, and (4) understanding of the mechanisms controlling hormone binding selectivity. Because hormone transport in man differs from animals in which most of the biological testing has been made, it is important to understand the nature of hormone-protein interactions with animal TTR, in particular rat which has been extensively studied. Therefore, the goal of this collaborative project is to extend the structural basis for the structure-activity correlations for thyroid hormone analogues, in particular for their binding affinity to rat transthyretin, a fully functional "variant" which is 85% homologous to human TTR. These aims will be accomplished by crystal structure determination of rat transthyretin and its complexes with thyroxine and its binding competitors. The detailed structural comparison of rat and human TTR will provide information relating the role of sequence substitutions in folding and transmission of conformational changes to the binding site. In summary, these studies will identify features essential to specific interactions in hormone binding to rat TTR and provide the basis for a better understanding of the hormone interactions required for molecular recognition and will also provide a model for the design of hormone antagonists which can be used for more effective hormonal control at the cellular level.